It is a stratospheric sum but it does, at least, include the return journey. Elon Musk, the billionaire American transport visionary, has suggested that the first, so far unnamed, passengers on his SpaceX flight round the moon will pay about $70m (£56m).

Musk says the journey, tentatively scheduled for 2018 on an untested Falcon Heavy rocket, will cover up to 400,000 miles, although the Apollo 13 crew, on their trip to the moon in 1970, were a record 248,655 miles from Earth, so this figure seems modest if anything. Either way, 400,000 miles (about 16 times the circumference of Earth) for £56m is equal to about £140 a mile, which is easier to fathom. But how does it compare with terrestrial journeys?

Tube: The closest you can get to matching the cost of lunar travel is on the London Underground. The shortest Tube journey is the 350 metres, from Covent Garden to Leicester Square. A cash ticket costs £4.90, which equates to almost £23 a mile, about a sixth of the cost of a trip to the moon and back – and a lot quicker.